1. Field
This invention pertains generally to the nondestructive examination of tubular specimens and, more particularly, to an inspection probe for the nondestructive examination of the structural integrity of heat exchanger tubing.
2. Related Art
In pressurized water reactor nuclear power plants, steam generators convert the thermal energy of water from the reactor coolant to steam to drive turbine electric generators. In order to transfer the heat while maintaining separation between the high pressure water that flows through the reactor core and the lower pressure water that is converted to steam, steam generators are constructed of thousands of small diameter tubes which provide a large surface area for heat transfer. The number of tubes in a steam generator range from about 3,000 to 15,000. Some steam generators utilize straight length tubes each about 60 feet long. Most of the steam generators are constructed of U-shaped tubing or long vertical sections with two 90° bends joined by a shorter horizontal length of tubing. During plant operation, the high pressure water that flows through the reactor core transports some amount of radioactive particles through the steam generators and some particles become deposited on the interior surface of the tubes. After plant operation, the steam generators become a source of radiation.
Periodic inspection with eddy current probes is widely utilized to ensure the structural integrity of steam generator tubing. Due to the elevated radiation fields, robotics and remote controlled motorized devices are used to position and translate eddy current probes. The cost of equipment, labor, plant down time, and the benefit of minimizing personnel radiation exposure make it highly desirable to optimize the performance and capability of eddy current inspection probes.
One problem with the prior art eddy current probes is that a single probe does not access all the tubes in a steam generator. The larger diameter probes used to inspect the majority of the tubes will not pass through the small radius bends in the tubing. To access the small radius bends, a small probe with less resolution may be used. Additionally, in order to inspect the entire tube, only one half of the tube may be accessed from one side of the steam generator and the second half of the tube may require access from the opposite end of the tube. To maximize productivity, this usually requires the use and disposal of additional probes.
A second problem with current eddy current probes is the probe centering mechanism. Typically, the probes are centered employing compliant pads that extend out radially at equally spaced circumferential locations around the probe. The relatively small surface area of contact between the probes and tube increases radial material loss due to wear. To compensate for wear, the pads are slightly oversized which increases friction between the tube and the pads. A further drawback of the probe centering pads is that each pad is compressed, the amount of deflection is independent from one pad to another pad. Side loads developed as the probe traverses bends in the tubing can adversely impact the centering of the probe. Additionally, the axial location of probe centering pads relative to the probe inspection coil can cause the coil to contact the tube surface as the probe traverses bends, which can adversely affect proper interpretation of the sensor signals.
Another problem currently experienced is probe electrical signal failures. While the cause of electrical failures can be ambiguous, controlling the amount of flexure of the probe is expected to decrease stress in the probe wiring and reduce electrical failures.
An additional difficulty currently encountered during eddy current inspection is in regard to inserting the probe into the tube. Typically, an eddy current probe is pushed into the tube by mechanical means such as wheels or belts which engage the probe's flexible cable. Probe friction with the tube, gravity, and rubbing of the flexible cable attached to the probe induce forces that oppose insertion of the probe. The flexible cable attached to the probe tends to buckle and the side loads imparted on the tube further increase the friction force opposing insertion. In many cases, the friction due to buckling will continue to increase the buckling load and further increase friction until it is not possible to insert the probe regardless of the force applied. Since the probe position is measured externally by encoding the flexible cable displacement, buckling of the flexible cable also causes loss of probe position accuracy.
As previously described, there are a number of problem areas with current eddy current inspection techniques. It is an object of this invention to provide a single eddy current probe to access the entire steam generator tubing including the smallest radius U-bends.
It is a further object of this invention to provide such a probe that has wheels to reduce friction in all areas of the tube including the point of insertion, diameter transitions, dents and bends.
It is an additional object of this invention to provide such a probe that will keep the sensor centered through diameter transitions, bends and other anomalies in the tube.
It is a further object of this invention to provide such an inspection probe that limits internal wire bending to enhance probe life.
It is an additional object of this invention to provide such a probe that has a dynamic seal that will enable an insertion force at the probe and move the probe along while maintaining it centered.
Additionally, it is an object of this invention to provide such a probe that has enhanced axial position accuracy.